Vapor-phase growth process is currently utilized in various industrial fields. Needless to say in the vapor-phase growth, advanced uniformities in thickness, composition and doping concentration of a film grown on the wafer over the entire surface thereof are an essential matter. Achievement of thermal uniformity in wafer heating is therefore recognized as the most important elementary technology as one means for realizing the aforementioned uniformities over the entire surface.
FIG. 1 is a sectional view showing an exemplary constitution of a general vapor-phase growth apparatus. As shown in FIG. 1, a vapor-phase growth apparatus 100 comprises a reaction furnace 1, a wafer holder 3 for disposing the wafer 2 thereon, a susceptor 4 for placing the wafer holder 3 thereon, a heater 5 disposed below the susceptor 4, a rotary mechanism 6 for supporting the wafer holder 3 and the susceptor 4 to allow them to rotate freely, a gas introducing duct 7 for supplying a source gas and a carrier gas therethrough, and a gas exhaust duct 8 for discharging the non-reacted gas, and the like.
FIG. 2 is an enlarged view for showing a detailed construction of the wafer holder 3, where (a) is a plan view, and (b) is a sectional view taken along the line A-A. In one surface of the wafer holder 3, a plurality of (six in FIG. 2) circular pocket holes 3a are formed for disposing the wafers 2 therein, to be arranged along a single circumference on the surface. The other surface of the wafer holder 3 is in contact with the susceptor 4.
The susceptor 4 herein is made of a material having a large coefficient of thermal conductivity (e.g., molybdenum) in order to uniformly transfer heat from the heater 5. It is also general to use graphite, molybdenum or the like, having a large coefficient of thermal conductivity for the wafer holder 3.
In the vapor-phase growth apparatus having such a structure described above, heat is transferred to the wafer 2 through the susceptor 4 and wafer holder 3 by heating the susceptor 4 from the lower side thereof by using the heater 5, to thereby heat the wafer 2 up to a predetermined temperature. Vapor-phase growth of a thin film is carried out by rotating the susceptor 4 at a predetermined number of rotation with the aid of a rotating mechanism 6 while uniformly supplying source gas and carrier gas, introduced through a gas introducing duct 7 toward the front surface of the wafer 2.
It was, however, found from an experiment of the present inventors that, in the aforementioned vapor-phase growth apparatus 100, the front surface temperature of the wafer 2 became lower than that of the wafer holder 3, and the temperature of the circumferential portion of the wafer 2 consequently became higher than that of the central portion of the wafer 2, by the effect of the temperature of the wafer holder 3. In other words, it was found to be difficult for the conventional vapor-phase growth apparatus 100 to form a thin film with a high uniformity over the entire surface of the wafer 2 by vapor-phase growth since in-plane temperature distribution of the wafer 2 could not be uniform.
The present invention has been developed in order to solve the aforementioned problems. An object of the invention is therefore to provide a vapor-phase growth apparatus which is capable of allowing a thin film to grow in a vapor phase so as to achieve a desirable uniformity over the entire surface of a wafer.